As is well known, the efficiency of the compressor section of a gas turbine is especially important in aircraft applications. Very close dimensional precision must be obtained in the parts, to optimize the aerodynamic performance of such machines. There have been many years of experience in making compressor parts and in working titanium alloys, of which they are often made. Thus the procedures for making compressor parts have been highly refined. However, a relentless demand by users for improved efficiency has continuously raised the degree of precision which must be obtained in compressor parts, and in particular the airfoil sections of such parts. Variations, defects and discrepancies which previously might be overlooked now must be eliminated.
To manufacture airfoils, metal is often first precision forged to nearly the final dimension and then the parts are finish machined. But during forging a thin oxidized layer, or an "alpha case" (alpha phase surface layer), is typically formed due to high temperature exposure. Chemical milling is then desirably used to generally remove this contaminated material. The basic process of chemical milling of titanium alloys is described in U.S. Pat. No. 2,981,609 to Snyder et al and in U.S. Pat. No. 3,745,079 to Cowles et al. See also "Chemical Machining (of Titanium Alloys)" in the Metals Handbook (American Society for Metals) Eighth Edition, Volume 3, pages 505-506.
An expeditious way of chemical milling is barrel finishing. Barrel finishing is employed in many industrial processes, e.g., plating, and essentially consists of placing a quantity of parts in a drum which rotates. But, one of the problems of such a procedure is that the parts tend to impact one another. Thus, when it was sought to apply barrel finishing to the chemical milling of titanium airfoils, small nicks of up to 0.25 mm deep were produced in the relatively fragile leading and trailing edges. Such a result is contradictory to an essential object of producing smooth edges, since in compressor airfoils the contour of the leading and trailing edges is quite important. As a result, the way titanium airfoils have been finished hereto when chemical milling has been used, has been to individually hold the parts in fixtures within the corroding medium. While this procedure has been technically satisfactory, in an effort to improve results and reduce costs, the work which led to the present invention was undertaken.
After chemical milling away the alpha case, mass abrasive finishing is usually used to smooth the leading and trailing edges. Of course, metal parts have been abrasively finished in barrels, usually vibrating vertical-axis barrels, for many years. In such a procedure, parts are immersed within a relatively large quantity of abrasive media, usually ceramic pellets. When motion is imparted to the barrel the pellets move against the surface of the metal objects and gently and uniformly machine them. But, such a procedure is not particularly useful in the desired general removal of alpha case from thin titanium airfoils of gas turbine engine compressors. The amount of finishing needed to remove a typical 0.05-0.12 mm of alpha case requires an undue time. And under such conditions there will be preferential material removal from the leading and trailing edges; while they are rounded there is excessive material removal and the overall chord length of the part will be unacceptably altered.